The discovery of a disease-carrying psyllid in Fresno, CA, has heightened concerns of California citrus growers that their groves are at risk from, and may soon be infected by, a devastating pest-borne bacterial plant disease known as citrus greening, or huanglongbing (which is known as HLB).

Diseased trees also were recently discovered in Mexico's Yucatan Peninsula, and there is concern that the disease could spread rapidly across Mexico's citrus-producing areas and then up into the United States.

The disease is harmless to humans but damages and eventually kills infected trees, and it ruins the flavor of the fruit or juice. "Once infected, there is no cure for a tree with citrus greening," according to the U.S. Department of Agriculture.

Fears that the disease could arrive in the California citrus belt by more direct means were intensified in late July when an insect testing positive for HLB was detected in luggage arriving in Fresno from India.

The disease has caused extensive damage and serious financial losses to the citrus industry in Florida but until recently had never been detected in either Mexico or California.

But the Asian citrus psyllid, a tiny aphid-like insect that can carry and transmit the disease, was detected at the southern tip of Baja California, Mexico, in 2007 and made its way up Baja to Tijuana, Mexico, in just under a year. Shortly thereafter, the psyllids crossed the border into California and have since been detected in numerous locations in San Diego and Imperial counties.

Wherever the psyllid goes, it is only a matter of time before the disease will show up, experts said.

"It's like Mary's little lamb," Ted Batkin, president of the California Citrus Research Board, wrote in an Aug. 19 e-mail to The Produce News. "Everywhere the pest goes, the disease is sure to follow. The only way to save California citrus is to control the psyllids."

State and federal agriculture officials, in cooperation with the California citrus industry, have been working aggressively to eradicate the psyllid infestations before the disease-causing organism comes on the scene.

Bob Blakely, director of industry affairs for California Citrus Mutual in Exeter, CA, told The Produce News that the discovery of HLB in Yucatan, although a long way from California, "is a big concern for us because it is the first time that HLB has been found on the mainland of Mexico." It is still some distance from Mexico's major citrus-producing area, but "we know that that area is already heavily infested with psyllid, so if an infected psyllid gets into that area, it just becomes a highway right up to the U.S. border."

Everywhere that the psyllid "has ever become established, it has only been a matter of time until the HLB shows up," Mr. Blakely said.

Once the disease infects a tree, "it has a latency of two to five years," he said. "It will lay dormant in the plant before the symptoms start to express themselves."

How long it will take the disease to travel from Yucatan to San Diego "is hard to determine," he said. "I don't know that you could put a timeline on it. We would hope that it wouldn't move into Southern California because of the eradication efforts that we've got ongoing now."

On the other hand, "we don't know that HLB isn't already in Southern California," even though it has not yet been detected, Mr. Blakely said. "We can't assume that some infected material" from a Florida nursery, for example, "did not find its way over here and get planted in California."

The detection of an infected psyllid in luggage in Fresno also underscores the seriousness of the possibility of "somebody leapfrogging it in here by bringing something in on an airplane," he said.

The infected pest in Fresno was found during "a routine inspection of bags at the FedEx terminal" by a dog that was sniffing packages for agricultural products. The dog located a bag of curry leaves in a duffle bag. The bag was luggage that had been separated from an airline passenger and was being delivered to its owner. Upon inspection, a dead psyllid and several live nymphs were found on the curry leaves. One of the nymphs tested positive for HLB. It was fortunate that the detection was made and the pest interdicted, said Mr. Blakely.

"That's what we have the ag dogs for," he said, adding that the pest could just as easily have slipped through undetected.

"This pest and disease can be catastrophic not only to local farms and farmers that we count on for our fresh, locally produced citrus, but also consumers' ability to grow citrus in their backyards," Mr. Batkin said in his e-mail.